This looks like a bug in EVOK. The /rest/wd interface should return it correctly, I'll fix this in the github repository shortly, but it may take a little while before it gets into a debian package.

Once the update appears on GitHub you may be able to update EVOK manually by overwriting your /opt/evok/lib/python2.7/site-packages/evok/evok.py with the new version, or you can wait until the next EVOK release.

It should be fixed in the latest Master development version on GitHub, but it may take a little while before the next release is done. You can test it out yourself by replacing the file as detailed above.